160 H o 158 H -o I 156^ 154- O -o CN Family size 3 4 1 1 1 1 Birth rank •£ -O "O *** r™ i_ — Figure 7. Variations with family size and birth rank in achieved height at various ages in boys followed from age 6 to 14. Source: Grant (37).r data to clarify several points. Taking advantage of the longitudinal nature of her material she showed that the differences in average (mean) measurements (at various ages) associated with family size are, in fact, the product of continuing slower rates of growth in children from larger families. The mean increments over a period of time, for example, between the sixth and tenth birthdays are shown in Table 6, are less in the children from larger families.